We have
a17 = a + 16d
and a10 = a + 9d
According to the given question,
a17 – a10 = 7
⇒ (a + 16d) – (a + 9d) = 7
⇒ a + 16d – a – 9d = 7
⇒ 7d – 7
⇒ d = 1
Hence, common difference is 1.
The first term of an AP is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.